Field-effect transistors (FETs) are often used for switching electrical loads on and off throughout a vehicle. A single vehicle may have several hundred FETs. The FETs are used in the activation and operation of lights, turn signals, wiper motors, video or audio systems, navigation systems, plus other various systems and devices. FETs are embedded within microcontrollers and electronic control modules. Over time and for various reasons shorts circuit conditions can arise that directly or indirectly affect the operation of the FETs. The short circuit conditions may arise, for example, due to changes in circuit connections, improper installation or repair of components, extended use of vehicle electronic systems, and degradation or deterioration in components. The short circuit conditions include “shorts” or inadvertent connections to a power source or a ground.
It has become apparent that many FETs are incapable of tolerating repeated high current caused by the short circuit conditions. High currents cause a FET to experience a rapid rise in junction temperature. This rapid rise in temperature and associated temperature differential between the junction and the rest of the silicon causes degradation to the FET. The degradation caused by the rapid rise in temperature tends to be worse at low ambient temperatures due to the larger temperature differential. When a short circuit condition exists, repeated cycling of a FET can result in damage to the FET and potentially render the FET and/or any associated electronic module coupled thereto to operate inappropriately or to be inoperative.
One known technique prevents the continued operation of a FET upon detection of a short circuit condition. The FET is switched off as quickly as possible after the short circuit is detected. Although this technique may prevent damage to the FET, it has associated disadvantages. One such disadvantage is the disablement of the FET in a false short circuit condition. A “false short circuit condition” refers to when there is a temporary spike of short duration in the operating temperature or current of a FET, which is improperly detected as a short circuit condition. Such a false condition can occur for various reasons. For example, when a capacitive or motor load is initially activated, it causes a large initial spike in current (in-rush); however, this current spike lasts for a short period of time and is considered normal. As another example, an intermittent open circuit on a capacitive/motor load can cause multiple in-rush currents that could be also detected as short circuits.
Thus, there exists a need for an improved FET short circuit intervention system that overcomes the disadvantages associated with hardware protection techniques currently in use.